Low formaldehyde emitting crosslinking agents, process of preparation and curable compositions thereof

ABSTRACT

Described is a liquid, partially alkoxymethylated melamine crosslinker composition having from about 2.6 to about 4.6 moles of combined formaldehyde per mole of melamine, from about 1.4 to about 4.4 moles of alkyl per mole of melamine, from about 1.3 to about 3.4 moles of NH per mole of melamine, a free formaldehyde level of less than about 0.5 weight percent based on 100 percent solids, and an N-methylol level of less than about 6.0 weight percent; processes for its preparation; a low formaldehyde emitting curable composition containing the crosslinker; as well as a method of coating a substrate and substrates so coated.

This is a continuation of application Ser. No. 08/488,987 filed on Jun.8, 1995 ABN.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to low free formaldehyde and low N-methylol groupcontaining melamine derived crosslinker compositions and to lowformaldehyde emitting thermosetting coating compositions based on thesecrosslinkers compositions, which coating compositions can providedurable, substantially defect-free films upon curing. The lowformaldehyde emitting thermosetting coating compositions in accordancewith the present invention contain, in their broadest concept, a lowfree formaldehyde and low N-methylol group containing partiallyalkoxymethylated high NH (high imino) melamine crosslinking agent and apolyfunctional material containing functional groups capable of reactingtherewith.

2. Description of Related Art

Currently, the majority of important industrial coatings includemelamine-formaldehyde crosslinked liquid systems. Such liquid coatingswhich employ partially or fully alkoxymethylated amino resincrosslinkers are in general well-known to those of ordinary skill in theart, as exemplified by numerous references including, for example, U.S.Pat. No. 4,081,426, U.S. Pat. No. 4,101,520, U.S. Pat. No. 4,129,681,U.S. Pat. No. 4,243,705, U.S. Pat. No. 4,276,212, U.S. Pat. No.4,330,458, U.S. Pat. No. 4,374,164, U.S. Pat. No. 4,433,143, U.S. Pat.No. 4,425,466, U.S. Pat. No. 4,873,298, U.S. Pat. No. 5,155,201,JP-A-51125102 and GB-A-2254328, all of which are incorporated herein byreference for all purposes as if fully set forth. These and otherreferences disclose a wide variety of backbone resins, catalysts,additives and amino resin crosslinkers therefor including, for example,tris- and tetra-substituted melamines for coatings applications. Thepresent invention is concerned with liquid amino resin crosslinkers and,in particular, with the alkoxymethyl substituted melamine derivedcrosslinkers.

A problem with melamine formaldehyde resins is that they emitformaldehyde during the curing process, creating potential health andsafety hazards and environmental pollution problems. Because of thehealth, safety and environmental concerns associated with large scaleuse of compositions containing melamine-formaldehyde crosslinkingagents, demand for lower formaldehyde emitting coatings has steadilyincreased in recent years providing an incentive to find a practicalsolution to the problem.

One common approach to solving this problem has been the use of low freeformaldehyde containing melamine derived crosslinking agents. Whileemploying low free formaldehyde crosslinkers has been generally helpfulto some extent, the improvement has been modest because preparation oflow free formaldehyde containing crosslinkers has been difficult andemission problems have persisted even when low free formaldehydecontaining crosslinkers are employed. Surprisingly, formaldehydeemissions during cure are greater than the actual free formaldehydelevels present in the starting coatings compositions. The origin of theexcess formaldehyde released during cure has not been completelyelucidated to date and, as a result, the problem of formaldehydedischarge into the environment continues to concern the coatingsindustry.

It has now been surprisingly discovered, in accordance with the presentinvention, that formaldehyde emission levels of well-knownmelamine-formaldehyde crosslinked systems can be reduced by replacingthe previous melamine formaldehyde crosslinkers with a certain class oflow free formaldehyde, low N-methylol and relatively high NH groupcontaining liquid alkoxymethylated melamine resins. Curable compositionscontaining these crosslinkers of the present invention cure rapidlyunder normal cure conditions to produce durable films without emittingunacceptably high levels of formaldehyde.

SUMMARY OF THE INVENTION

As indicated above, the present invention is a liquid, partiallyalkoxymethylated melamine crosslinker composition having from about 2.6to about 4.6 moles of combined formaldehyde per mole of melamine, fromabout 1.4 to about 4.4 moles of alkyl per mole of melamine, from about1.3 to about 3.4 moles of NH per mole of melamine, a free formaldehydelevel of less than about 0.5 weight percent based on 100 percent solids,and an N-methylol level of less than about 6.0 weight percent,preferably less than about 5.0 weight percent, and more preferably lessthan about 4.0 weight percent, based on 100 percent solids.

The present invention is further a low formaldehyde emitting curablecomposition which, when cured, produces a formaldehyde emission of lessthan about 7.5 weight percent based on the weight of the crosslinker,comprising the above-described crosslinker of the present invention anda polyfunctional material reactive therewith. These compositions arerapid curing but do not emit unacceptably high levels of formaldehyde.

The present invention is still further a process for preparing thecrosslinkers of the invention said process comprising the step ofcontacting a liquid partially alkoxymethylated melamine crosslinker anda reagent selected from the group consisting of an amino groupcontaining compound and an etherification agent, as described in furtherdetail below.

The present invention is still further a method of coating a substrateby applying onto a substrate the low formaldehyde emitting curablecompositions of the invention and then heat curing the so appliedcoating.

Finally, the present invention is crosslinked film or coated articleprepared by the above method of coating. Such crosslinked films possessgood hardness, impact resistance and solvent resistance, and aresubstantially surface defect-free with desirable appearancecharacteristics such as gloss and lack of pinholes or blisters.

These and other features and advantages of the present invention will bemore readily understood by those of ordinary skill in the art from areading of the following detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The Melamine Crosslinking Agent

The novel melamine crosslinking agents of the present invention, asindicated above, have from about 2.6 to about 4.6 moles of combinedformaldehyde per mole of melamine, from about 1.4 to about 4.4 moles ofalkyl per mole of melamine, from about 1.3 to about 3.4 moles of NH permole of melamine, a free formaldehyde level of less than about 0.5weight percent based on 100 percent solids, and an N-methylol level ofless than about 6.0 weight percent, preferably less than about 5.0weight percent, and more preferably less than about 4.0 weight percent,based on 100 percent solids.

In one preferred embodiment, referred to herein as very high NHcrosslinkers, the combined formaldehyde to melamine molar ratio ispreferably in the range of from about 2.6 to about 3.4, and morepreferred ratio is in the range of from about 3.0 to about 3.4; thealkyl to melamine molar ratio is preferably in the range of from about1.4 to about 3.2, and more preferably in the range of from about 2.4 toabout 2.8; and the NH to melamine molar ratio is preferably in the rangeof from about 1.8 to about 3.4, and more preferably in the range of fromabout 2.0 to about 2.8.

In a second preferred embodiment, referred to herein as moderately highNH crosslinkers, the combined formaldehyde to melamine molar ratio ispreferably in the range of from about 3.6 to about 4.6, and morepreferably is in the range of from about 3.8 to about 4.2; the alkyl tomelamine molar ratio is preferably in the range of from about 2.4 toabout 4.4, and more preferably in the range of from about 2.6 to about3.8; and the NH to melamine molar ratio is preferably in the range offrom about 1.3 to about 2.4, and more preferably in the range of fromabout 1.4 to about 1.8. As an especially preferred such moderately highNH crosslinker may be mentioned one wherein the combined formaldehyde tomelamine molar ratio in the range of from about 3.8 to about 4.0, thealkyl to melamine molar ratio in the range of from about 2.6 to about3.0, and an NH to melamine molar ratio in the range of from about 1.4 toabout 1.6.

When cured, the liquid partially alkoxymethylated melamine crosslinkercompositions of the invention produce a formaldehyde emission of lessthan about 7.5 weight percent based on the weight of the crosslinker,and in some cases, particularly in cases such as those prepared by aprocess which includes both treatment with an amino group containingcompound and etherification steps as described below, they produce aformaldehyde emission of less than about 4.3 weight percent based on theweight of the crosslinker.

The liquid partially alkoxymethylated melamine crosslinkers of thepresent invention preferably comprise at least 30 weight percentmonomeric species, the balance being s complex mixture of productsincluding, for example, monomeric bis-, tris-, and tetra-substitutedalkoxymethylated melamine derivatives, N,N-'-bis-alkoxyalkyl melamines,N,N',N"-tris-alkoxyalkyl melamines, and N,N', N",N"-tetrakis-alkoxyalkylmelamines, oligomers derived from the di-, tri-, and tetra-substitutedderivatives, and other variations recognizable by those of ordinaryskill in the relevant art. The preferred crosslinker for use in thepresent invention preferably comprises from about 40 wt. % to about 65wt. % monomer. The term "Wt. % monomer" for the purposes of the presentinvention is equated with the area percent of the monomeric portion of acurve determined by analysis of the partially alkoxymethylated melaminecrosslinker by well-known high performance size exclusion chromatographytechniques.

Each alkoxy group of the present partially alkoxymethylated melamines isindependently based on an alkyl group preferably selected from the groupconsisting of linear, branched and cyclic alkyls of 1 to 20 carbonatoms. Especially preferred alkyl groups are lower alkyls of 1 to 8carbon atoms including, for example, methyl, ethyl, 1-propyl, 2-propyl,1-butyl, 2-butyl, iso-butyl, tert-butyl, 1-pentyl, 1-hexyl, cyclohexyland 2-ethyl-1-hexyl. The most preferred alkyls are methyl, ethyl andbutyl groups. Also preferred are crosslinkers having mixed alkyl groups,such as mixed methyl and butyl groups.

Curable Compositions

The low formaldehyde emitting curable composition of the invention ischaracterized as being a low formaldehyde emitting composition which,when cured, produces a formaldehyde emission of less than about 7.5weight percent, preferably less than about 6.5 weight percent, and morepreferably less than about 4.3 weight percent, based on the weight ofthe crosslinker.

In general, the curable compositions of the present invention can bebased on any well-known melamine resin crosslinked composition in whichall or a portion of the existing melamine resin is replaced with thecrosslinker composition of the present invention. For generaldescription purposes, these curable composition may be characterized ascomprising (i) the crosslinker compositions of the present invention and(ii) a polyfunctional material capable of reacting therewith.

The polyfunctional material containing a functionality capable ofreacting with the liquid partially alkoxymethylated melaminecrosslinkers of the invention under normal coatings cure conditions(generally between about 25° C. to about 250° C.) are well knownbackbone resins widely used in amino resin crosslinked coatings. Theseresins contain at least two reactive functionalities each preferablyindependently selected from the group consisting of hydroxy, carboxy,amino, amido, carbamato, mercapto, and a group convertible thereto. Thepreferred polyfunctional materials are hydroxyfunctional orcarboxyfunctional acrylic or polyester backbone resins, withhydroxyfunctional materials being especially preferred. Illustrativeexamples of polyfunctional materials include acrylic resins which may beobtained by the copolymerizationof acrylic or methacrylic esters withhydroxyfunctional acrylic or methacrylic esters such as hydroxyethylacrylate or methacrylate, optionally with simultaneous use of additionalvinyl compounds such as, for example, styrene. Illustrative examples of,polyfunctional materials also include polyester resins which may beobtained, for example, by the reaction of polycarboxylic acids withexcess quantities of polyhydric alcohols. Suitable hydroxyfunctionalresins also include epoxy or polyurethane prepolymers, alkyds, and thelike.

In general, such resins may have pendent or terminal hydroxylfunctionalities and preferably have the following characteristics:weight average molecular weights (Mw) of from about 750 to about 7000,and more preferably from about 2000 to about 5000; and hydroxyl numbersof from about 20 to about 100 mg KOH/g resin, more preferably from about25 to about 60 mg KOH/g resin, and especially from about 25 to about 40mg KOH/g resin. For waterborne coatings applications, materials havingmuch higher molecular weights are generally preferred. Other suitablehydroxyl functional resins will be readily recognized by those ofordinary skill in the art.

Commercially available examples of suitable polyfunctional hydroxy groupcontaining materials include JONCRYL® 500 acrylic resin (S. C. Johnson &Sons, Racine, Wis.), ACRYLOID® AT-400 acrylic resin (Rohm & Haas,Philadelphia, Pa.), CYPLEX® 1531 polyester resin (Cytec Industries, WestPaterson, N.J.), CARGILL® 3000 and 5776 polyester resins (Cargill,Minneapolis, Minn.), TONE® polyester resin (Union Carbide, Danbury,Conn.), K-FLEX® XM-2302 and XM-2306 resins (King Industries, Norwalk,Conn.), CHEMPOL® 11-1369 resin (Cook Composites and Polymers, PortWashington, Wis.), JONCRYL® 540 acrylic emulsion polymer (S. C. Johnson& Sons, Racine, Wis.), RHOPLEX® AC-1024 acrylic emulsion resin (Rohm &Haas, Philadelphia, Pa.), XC® 4005 water reducible acrylic resin (CytecIndustries, West Paterson, N.J.), CRYLCOAT® 3494 solid hydroxyterminated polyester resin (UCB CHEMICALS USA, Smyrna, Ga.), RUCOTE® 101polyester resin (Ruco Polymer, Hicksville, N.Y.), JONCRYL® SCX-800-A andSCX-800-B hydroxyfunctional solid acrylic resins (S. C. Johnson & Sons,Racine, Wis.), and the like. Examples of carboxyfunctional resinsinclude CRYLCOAT® solid carboxy terminated polyester resin (UCBCHEMICALS USA, Smyrna, Ga.), RUCOTE® 101 polyester resin (Ruco PolymerCorporation, Hicksville, N.Y.), ALFTALAT® AN 745 hydroxyfunctionalpolyester resin (Hoechst Corporation) and ARAKOTE® 3010 carboxyterminated polyester (Ciba-Geigy Corporation, Ardsley, N.Y.). Forsolvent borne coatings, liquid polyfunctional hydroxy group containingmaterials are preferred; however, a solid polyfunctional hydroxy groupcontaining material may be used in cases when the solids are soluble inthe solvent used in a particular formulation. Resins containing amino,amido, carbamato, or mercapto groups, including groups convertiblethereto, may be prepared by known methods including copolymerizing asuitably functionalized monomer with a comonomer capable ofcopolymerizing therewith.

The curable composition of the present invention may further comprise acure catalyst, which again are in general well known to those ofordinary skill in the art. The cure catalysts usable in the presentinvention include, for example, sulfonic acids, aryl, alkyl, and aralkylacid phosphates, aryl, alkyl, and aralkyl acid pyrophosphates,carboxylic acids, sulfonimides, mineral acids, Lewis acids,organometallic compounds and a mixture thereof. Of the above acids,sulfonic acids are preferred. Examples of the sulfonic acids includebenzenesulfonic acid, para-toluenesulfonic acid, dodecylbenzenesulfonicacid, naphthalenesulfonic acid, dinonyinaphthalenedisulfonic acid, and amixture thereof. Examples of the aryl, alkyl, and aralkyl phosphates andpyrophosphates include phenyl, para-tolyl, methyl, ethyl, benzyl,diphenyl, di-para-tolyl, di-methyl, di-ethyl, di-benzyl,phenyl-para-tolyl, methyl-ethyl, phenyl-benzyl phosphates andpyrophosphates. Examples of the carboxylic acids include benzoic acid,formic acid, acetic acid, propionic acid, butyric acid, dicarboxylicacids such as oxalic acid, fluorinated acids such as trifluoroaceticacid, and the like. Examples of the sulfonimides include dibenzenesulfonimide, di-para-toluene sulfonimide, methyl-para-toluenesulfonimide, dimethyl sulfonimide, and the like. Examples of the mineralacids include nitric acid, sulfuric acid, phosphoric acid,poly-phosphoric acid, and the like. Examples of the Lewis acids includeboron and aluminum halides. Examples of the organometallic compoundsinclude organotin compounds such as dibutyltin di-2-ethylhexoate,dibutyltin diisooctyl maleate, dibenzyltin di-2-ethylhexoate, dibutyltindilaurate, dimethyltin dilaurate, tetrabutyl diacetoxy distannoxane,tetramethyl diacetoxy distannoxane, tetrapropyl diacetoxy distannoxane,dibutyltin dichloride, and the like.

The curable composition of the present invention may also contain, as anoptional ingredient, a medium such as a liquid medium to aid the uniformapplication and transport of the curable composition. Any or all of theingredients of the curable composition may be contacted with the liquidmedium. Moreover, the liquid medium may permit formation of adispersion, emulsion, invert emulsion, or solution of the ingredients ofthe curable composition. Particularly preferred is a liquid medium whichis a solvent for the curable composition ingredients. Suitable suchsolvents include aromatic hydrocarbons, aliphatic hydrocarbons,halogenated hydrocarbons, ketones, esters, ethers, amides, alcohols,water, compounds having a plurality of functional groups such as thosehaving an ether and an ester group, and a mixture thereof.

The low formaldehyde emitting curable compositions of the invention mayalso include varying amounts of other conventional additives well knownto those of ordinary skilled in the art of coating, as exemplified bythe previously incorporated references. Included among these areadditives such as fillers, antioxidants, ultraviolet light stabilizers,pigments such as, for example, TiO₂ pigment, flow control agents,plasticizers, mold release agents, corrosion inhibitors, and the like.

The relative amounts of the components in accordance with the presentinvention are not in general critical. Any proportion of ingredients maybe employed that results in a curable coating composition capable ofproviding a durable, substantially surface defect-free finish aftercuring. In general, the ratio of the polyfunctional material to thecrosslinking agent is in the range of from about 99:1 to about 0.5:1.The weight percent of the cure catalyst, if present, is generally in therange of from about 0.01 to about 3.0 wt. % based upon the combinedweight of the backbone resin and crosslinker.

Preferably, the amount of crosslinker employed is typically in the rangeof from about 3 to about 50 wt. %, and more preferably in the range offrom about 10 to about 30 wt. %, of the binder which, for the purposesof the present invention, is the combined weight of the crosslinker andthe polyfunctional material (backbone resin). Conversely, the preferredamount of the polyfunctional material employed is typically in the rangeof from about 70 to about 97 wt. %, and more preferably in the range offrom about 80 to about 85 wt. %, of the binder.

Process of Preparation

The process for preparing the crosslinkers of the present inventioncomprises the step of contacting a liquid partially alkoxymethylatedmelamine resin and a reagent selected from the group consisting of anamino group containing compound and an etherification agent. Thecontacting is carried out at a temperature and length of time sufficientto produce the crosslinkers of the invention which contain less thanabout 0.5 weight percent free formaldehyde and less than about 6.0weight percent N-methylol groups, both numbers being based on 100percent solids. Preferably, the N-methylol group level is less thanabout 5.0 weight percent and, more preferably, less than about 4.0weight percent.

Typically, the starting material is a liquid partially alkoxymethylatedmelamine derivative having 2.6 to 4.6 moles of combined formaldehyde permole of melamine, 1.4 to 4.4 moles of alkyl per mole of melamine, 1.3 to3.4 moles of NH per mole of melamine and relatively high N-methylollevels (e.g., greater than 6.0 wt. % based on 100 percent solids) and/orfree formaldehyde levels (e.g., greater than 0.5 wt. % based on 100percent solids), and includes resins such as CYMEL® 322 crosslinker,CYMEL® 323 crosslinker, CYMEL® 324 crosslinker, CYMEL® 325 crosslinker,CYMEL® 327 crosslinker, CYMEL® 350 crosslinker, CYMEL® 370 crosslinker,CYMEL 373 crosslinker, CYMEL® 380 crosslinker, CYMEL® 385 crosslinker,and CYMEL® 1158 crosslinker, all products of Cytec Industries, Inc.,West Paterson, N.J. A brief description and the degree ofoligomerization (average number of triazine units per molecule) aredepicted below:

    __________________________________________________________________________    Methoxymethylmelamines                                                        CYMEL ® 322                                                                             high methylol/imino content, moderate degree of                               oligomerization (1.75)                                          CYMEL ® 323                                                                             high methylol/imino content, moderate degree of                               oligomerization (1.8)                                           CYMEL ® 325                                                                             high methylol/imino content, higher degree of                                 oligomerization                                                               (2.3)                                                           CYMEL ® 327                                                                             high methylol/imino content, moderate degree of                               oligomerization (1.75)                                          CYMEL ® 350                                                                             high methylol/imino content, moderate degree of                               oligomerization (1.55)                                          CYMEL ® 370                                                                             high methylol/imino content, higher degree of                                 oligomerization                                                               (2.5)                                                           CYMEL ® 373                                                                             high methylol/imino content, higher degree of                                 oligomerization                                                               (2.0)                                                           CYMEL ® 380                                                                             high methylol/imino content, higher degree of                                 oligomerization                                                               (2.5)                                                           CYMEL ® 385                                                                             high methylol/imino content, higher degree of                                 oligomerization                                                               (1.67)                                                          Butoxymethyl Melamines                                                        CYMEL ® 1158                                                                            n-butoxy, high methylol/imino content, high degree of                         oligomerization (2.7)                                           Mixed Alkoxymethyl Melamines                                                  CYMEL ® 324                                                                             methoxy/n-butoxy, high methylol/imino content, higher                         degree                                                                        of oligomerization (2.3)                                        __________________________________________________________________________

Other suitable liquid partially alkoxymethylated melamine derivativeswhich are not commercially available may be easily prepared frommelamine, formaldehyde, and an alcohol at the desired proportions usingwell known procedures such as those described in, for example,previously incorporated U.S. Pat. No. 4,101,520, U.S. Pat. No. 4,433,143and U.S. Pat. No. 4,425,466, and well known modifications thereof. Forexample, melamine may be used as a starting material to produce apartially alkoxymethylated derivative by reaction with an etherificationagent, which can be further reacted (for example, in multiple steps)with the etherification agent and/or amino group containing compound.One preferred process is, indeed, a triple etherification of melamine asdemonstrated in an example below.

In the practice of the process of the invention, the starting material(containing relatively high levels of N-methylol groups and varyinglevels of free formaldehyde) and a reagent selected from the groupconsisting of an amino group containing compound and an etherificationagent are preferably contacted at a temperature in the range of fromabout 25° C. to about 120° C., and for a period in the range of about 10minutes to about 10 hours, to produce a liquid partiallyalkoxymethylated melamine crosslinker containing less than about 0.5weight percent free formaldehyde based on 100 percent solids and lessthan about 6.0 weight percent N-substituted methylol groups based on 100percent solids. If desired, the procedure may be repeated to furtherreduce the free formaldehyde and the N-substituted methylol grouplevels.

The amino group containing compound referred to above is preferablyselected from the group consisting of amines, aminotriazines, amides,sulfonamides, phosphoramides, ureas, carbamates, ammonia and a mixturethereof. Amines which are effective in reducing the free formaldehydeand the N-substituted methylol group levels include ammonia andcompounds which contain primary and/or secondary amine groups. A mixturecontaining a plurality of amines may also be used. The amino groupcontaining compound may monofunctional or it may be polyfunctional andit may be monomeric or polymeric. Polyfunctional or polymeric aminogroup containing compounds which are relatively insoluble in the resinare preferred.

The etherification agent preferably comprises an alcohol and an acidcatalyst. Alcohols corresponding to the alkyls referred to above, thepreferred being alcohols of 1 to 8 carbon atoms, and especiallymethanol. A wide variety of acids are usable in the etherificationreaction, including the acid catalysts mentioned above, and preferablythe strong mineral acids.

The process may further require filtering insoluble materials formedduring the reaction and/or it may require stripping volatiles includingany unreacted volatile alcohol such as methanol.

While either contacting with an amino group containing compound orcontacting with an etherification agent is in and of itself sufficientin reducing the free formaldehyde and N-substituted methylol grouplevels to very low levels, it has been discovered that by combining thetwo approaches, very low free formaldehyde and N-substituted methylolgroup levels are readily achieved. For example, when the amino groupcontaining compound is first contacted and thereafter the etherificationagent is contacted, crosslinkers containing less than about 0.3 weightpercent free formaldehyde and less than about 4.0 weight percentN-substituted methylol groups are routinely obtained.

Method of Coating

The method of coating of the present invention produces a formaldehydeemission of less than about 7.5 weight percent based on the weight ofthe crosslinker. The method generally comprises:

(a) applying onto a substrate a curable composition as described above;and thereafter

(b) heat-curing the so-applied curable composition.

In the practice of the coating method of the present invention, curablecompositions containing a liquid medium such as a solvent may beadvantageously used. Contacting may be carried out by dipping, spraying,padding, brushing, flowcoating, electrocoating or electrostaticspraying. Typically, the solvent is allowed to partially evaporate toproduce a uniform coating on the substrate. Thereafter, the compositionsmay be fully cured by further application of heat at a temperature inthe range of from about 25° C. to about 300° C. and preferably at atemperature in the range of from about 80° C. to about 220° C. for aperiod of time preferably in the in the range of about 5 minutes toabout 30 minutes to obtain a fully cured coating, film, or objectprepared in accordance with the method of the invention.

Uses of the Curable Compositions

The heat cured compositions of this invention may be employed ascoatings in the general areas of coatings such as original equipmentmanufacturing (OEM) including automotive coatings, general industrialcoatings including industrial maintenance coatings, architecturalcoatings, coil coatings, can coatings, wood coatings, and lowtemperature cure automotive refinish coatings. They are usable ascoatings for wire, appliances, automotive parts, furniture, pipes,machinery, and the like. Suitable surfaces include metals such as steeland aluminum, plastics, wood, and glass. The low formaldehyde emittingcompositions of the present invention are also well suited for use torefinish automotive parts and to coat heat sensitive substrates such aswood.

The crosslinkers of the present invention may also be used as bindersfor non-wovens, as textile treatment agents for permanent presstextiles, as coating insolublizers for gellation of starch in paper, andas colloidal wet and dry strength agents in paper manufacture. Inaddition to coatings, curable compositions containing them may be usedin adhesives, decorative laminated boards and crosslinked moldedarticles.

The examples which follow are intended to be illustrative of certainpreferred embodiments of the invention and are not to be construed tolimit the invention in any manner. In the following examples, allamounts are expressed as parts by weight.

EXAMPLE 1

PART A: Synthesis of a crosslinker of the invention from melamine viatriple methylation (etherification)

Melamine (1 mole) was reacted with formaldehyde (4.4 moles) at pHgreater than 8.0 between 75°-80° C. for 5 minutes. Methanol (320 g, 10moles) was slowly added over 30 minutes between 75° to 64° C., and afirst methylation was then carried out at 64°-65° C. and pH=6.0 for 20minutes. 416 g of methanol was then added, and a second methylation wascarried out at 48° C. and pH=5.1 with for 70 minutes. A sample waswithdrawn. The monomer content of the withdrawn sample was 55 wt. % andOCH3/CH2 ratio was 0.67 by Nuclear Magnetic Resonance spectroscopy(NMR). Subsequently, another 416 g of methanol was added, and a thirdmethylation was carried out at 48° C. and pH=4.2 for 60 minutes to givea resin with properties listed in Table 1.

PART B: Synthesis of a crosslinker of the invention by re-methylation ofCYMEL® 325 resin

382 g of a resin, obtained by removing the solvent from CYMEL® 325resin, was mixed with 554 g of methanol. The pH was adjusted to 2.3 with48 g of a concentrated nitric acid, and the reaction temperature was 31°C. After 15 minutes, the reaction was stopped with a caustic solution(20%, 120 g) having a pH of 9.5. Properties of the resulting resin arelisted below in Table 1.

                  TABLE 1                                                         ______________________________________                                                                       From                                                     CYMEL ® 325                                                                         From PART A                                                                              PART B                                         ______________________________________                                        Free HCHO (wt %)                                                                          0.52        0.06       0.06                                       --CH2OH (wt %)                                                                            8.34        5.5        3.5                                        Foil solids (wt %)                                                                        77.1        82.3       86.2                                       OCH.sub.3 /CH.sub.2 Mole Ratio                                                            0.71        0.78       0.84                                       Viscosity   --          Z1         Z4-                                        Total Nitrogen (wt %)                                                                     21.5        23.6       24.15                                      Total HCHO (wt %)                                                                         29.8        31.3       33.4                                       Monomer (wt %)                                                                            42          50         41                                         Molecular Formula                                                                         MF.sub.3.8 Me.sub.2.7                                                                     MF.sub.3.7 Me.sub.2.9                                                                    MF.sub.3.8 Me.sub.3.2                      Molecular Weight                                                                          278         278        285                                        Methylol/Triazine                                                                         1           0.6        0.4                                        Molar Ratio                                                                   Moles NH/triazine                                                                         1.6         1.8        1.6                                        ______________________________________                                    

EXAMPLE 2

PART A. Synthesis of a crosslinker of the invention by treatment ofCYMEL® 327 resin with ammonium nitrate (ammonia source)

Ammonium nitrate (10 g) in 10 ml of water was slowly added to a CYMEL®327 resin (100 g) at 50° C. and pH=9.1. pH was maintained with a causticsolution during the addition (20%, 27 g). The solution was kept underthese conditions for 6 hrs., and the volatiles were removed underreduced pressure while maintaining the pH between 10.0 to 10.5. Afteraddition of isobutanol, the insolubles were filtered. The properties ofthe modified resin are listed in Table 2.

B. Synthesis of a crosslinker of the invention by treatment of CYMEL®327 resin with aqueous ammonia

CYMEL® 327 resin (100 g) and a 30 wt. % solution of aqueous ammonia (17g) were mixed and heated to 50° C., and then the pH was adjusted to 9.1with concentrated HNO3 (3.2 g). The solution was kept under theseconditions for 6 hrs., the volatiles were removed under reduced pressurewhile maintaining the pH between 10.0 to 10.5 then, after addition ofisobutanol, the insolubles were filtered. The properties of the modifiedresin are listed in Table 2.

                  TABLE 2                                                         ______________________________________                                                                       From                                                     CYMEL ® 327                                                                         From PART A                                                                              PART B                                         ______________________________________                                        Free HCHO (wt %)                                                                          0.33        0.12       0.23                                       --CH2OH (wt %)                                                                            11.63       4.7        4.4                                        Foil solids (wt %)                                                                        89.5        87.4       85                                         OCH.sub.3 /CH.sub.2 Mole Ratio                                                            0.77        0.76       0.82                                       Viscosity   Z4+         --         Z1-                                        Total Nitrogen (wt %)                                                                     24          24.2       24.7                                       Total HCHO (wt %)                                                                         36.1        35.3       34.3                                       Monomer (wt %)                                                                            62          60         61                                         Molecular Formula                                                                         MF.sub.4.2 Me.sub.3.2                                                                     MF.sub.4.1 Me.sub.3.1                                                                    MF.sub.3.9 Me.sub.3.3                      Molecular Weight                                                                          297         292        289                                        Methylol/Triazine                                                                         1.3         0.53       0.52                                       Molar Ratio                                                                   Moles NH/triazine                                                                         1.3         1.6        1.6                                        ______________________________________                                    

EXAMPLE 3

Synthesis of a crosslinker of the invention by treatments of CYMEL® 327resin with ion-exchange resins

PART A. With A-305 ion exchange resin

CYMEL® 327 resin (20 g) was allowed to react with A-305 ion exchangeresin beads (40 ml) at pH=9 in refluxing methanol (80 ml) for a periodof 2.5 hrs (A-305 is a product of Sybron Chemicals, Inc., Birmingham,N.J., and may be prepared by the reaction of an epoxide and apolyamine). The mixture was cooled to ambient temperature, the beadswere filtered, the methanol was evaporated and isobutanol was added. Theproperties of the modified resin are listed in Table 3.

Part B. With XR-374 ion exchange resin

CYMEL® 327 resin (20 g) was allowed to react with XR-374 ion exchangeresin beads (40 ml) at pH=8.4 in refluxing methanol (80 ml) for 5 hrs(XR-374 is a product of Sybron Chemicals, Birmingham, N.J., and may beprepared from divinylbenzene and N,N'-dimethyl-3-aminopropylacrylamide). The mixture was cooled to ambient temperature, the beadswere filtered, the methanol was evaporated and isobutanol was added. Theproperties of the modified resin are listed in Table 3.

                  TABLE 3                                                         ______________________________________                                                                       From                                                     CYMEL ® 327                                                                         From PART A                                                                              PART B                                         ______________________________________                                        Free HCHO (wt %)                                                                          0.42        0.49       0.07                                       --CH2OH (wt %)                                                                            10.3        5.2        3.99                                       Foil solids (wt %)                                                                        90.9        82.1       74.8                                       OCH.sub.3 /CH.sub.2 Mole Ratio                                                            0.77        0.85       0.84                                       Monomer (wt %)                                                                            62          61         60                                         ______________________________________                                    

EXAMPLE 4

PART A. Synthesis of a crosslinker of the invention by treatment CYMEL®327 resin with XR-374 ion exchange resin followed by etherification withmethanol

200 g of a resin, obtained by removing the solvent from CYMEL® 327resin, was allowed to react with XR-374 ion exchange resin beads (200ml) at pH=8.0 in refluxing methanol (278 ml) for 2 hrs. The mixture wascooled to ambient temperature, then the beads were filtered. The pH waslowered to 2.5 with concentrated nitric acid. After 15 minutes, the pHwas raised to 9.5 with caustic, the methanol was removed under reducedpressure, isobutanol was added and the insolubles were filtered. Theproperties of the modified resin are listed in Table 4.

PART B. Synthesis of a crosslinker of the invention by reetherificationof CYMEL® 327 resin with methanol

The pH of a solution of 200 g of a resin, obtained by removing thesolvent from CYMEL® 327 resin, in methanol (278 ml) was lowered to 2.5with concentrated nitric acid at room temperature. After 15 minutes, thepH was raised to 9.5 with caustic, the methanol was removed underreduced pressure, isobutanol was added and the insolubles were filtered.The properties of the modified resin are listed in Table 4.

                  TABLE 4                                                         ______________________________________                                                                       From                                                     CYMEL ® 327                                                                         From PART A                                                                              PART B                                         ______________________________________                                        Free HCHO (wt %)                                                                          0.43        0.2        0                                          --CH2OH (wt %)                                                                            8.2         2.3        4.28                                       Foil solids (wt %)                                                                        87.8        74.5       79.3                                       OCH.sub.3 /CH.sub.2 Mole Ratio                                                            0.74        0.89       0.86                                       Monomer (wt %)                                                                            65          65         61                                         Molecular Formula                                                                         MF.sub.4.0 Me.sub.3.0                                                                     --         MF.sub.4.0 Me.sub.3.5                      Molecular Weight                                                                          288         --         295                                        Methylol/Triazine                                                                         1           0.29       0.54                                       Molar Ratio                                                                   Moles NH/triazine                                                                         1.7         --         1.6                                        ______________________________________                                    

EXAMPLE 5

Synthesis of a crosslinker of the invention by remethylation of CYMEL®322 Resin

The pH of a solution of 296 g of a resin, obtained by removing thesolvent from CYMEL® 322 resin, in methanol (832 g) was adjusted to 3.77with a conc. nitric acid (17.8 ml). The reaction was kept at 35° C. for50 minutes and the pH was adjusted to 9.5 with a caustic (25%, 35 ml).The methanol was then removed under reduced pressure, isopropanol (20 g)was added and the insolubles were filtered. The properties of theunmodified and modified resins are summarized in Table 5.

                  TABLE 5                                                         ______________________________________                                                       Unmodified                                                                            Modified                                               ______________________________________                                        Pan solid (wt %) 71.8      83.9                                               Free HCHO (wt %) 0.9       0                                                  --CH2OH (wt %)   11        3                                                  OCH.sub.3 /CH.sub.2 Mole Ratio                                                                 0.5       0.80                                               Monomer (wt %)   65        60                                                 ______________________________________                                    

EXAMPLE 6

Cure response and formaldehyde emission: Water borne application

Formulation:

Joncryl® 540/Crosslinker of EXAMPLE 2 Weight ratio: 70/30

Solids: 45 wt. %

Surfynol® 104: 1.6 wt. % on total resin solids (TRS)

Methoxypropanol: 8.5 wt. % on TRS

Wire Cater: #52

Substrate: B1000 CRS

Solvent Flash: 15 min.

Cure: 30 min.

HCHO emission: 150° C./30 min. on glass, humid air, 15 min. flash

Joncryl® 540 acrylic resin (S. C. Johnson % Sons); Surfynol® 104Surfactant (Air Products); B-1000 panels (Advanced Coating Technology).

                  TABLE 6                                                         ______________________________________                                                                      Crosslinker of                                                     Crosslinker of                                                                           EXAMPLE                                                    CYMEL ®                                                                           EXAMPLE 2  2-B                                                        327     PART A     PART B                                          ______________________________________                                        100  C.                                                                       Mils         1.25      1.37       1.25                                        KHN          7.0       7.2        7.9                                         Pencil       HB-F      HB-F       HB-F                                        MEK          200       200        200                                         % Removed    1%        1%         1%                                          125° C.                                                                Mils         1.24      1.22       1.22                                        KHN          11.7      12.1       12.4                                        Pencil       H-2H      H-2H       H-2H                                        MEK          200       200        200                                         % Removed    slight scratch                                                                          slight scratch                                                                           slight scratch                              150° C.                                                                Mils         1.31      1.22       1.22                                        KHN          13.2      12.4       12.9                                        Pencil       H-2H      H-2H       H-2H                                        MEK          200+      200+       200+                                        % Removed    No Mar    No Mar     No Mar                                      % HCHO Emission (%)                                                                        2.9       1.9        1.7                                         (wt % emitted HCHO                                                            based on cured film)                                                          % HCHO Emission                                                                            9.7       6.3        5.7                                         (wt % emitted HCHO                                                            based on crosslinker)                                                         ______________________________________                                    

EXAMPLE 7

Cure response and formaldehyde emission: Solvent borne applicationFormulation: Acryloid® AT 400/Crosslinker of Example 2 Weight ratio:70/30 Solids: 66% n-BuOH: 20% on TRS Solvent: 11.4% Xylene on TRS Cure:30 min. Cater: #34 for 125 and 150° C./30 min. cure and #52 for 100°C./30 min. cure Acryloid® AT-400 acrylic resin (Rohm and Haas Co.,Philadelphia, Pa.).

                  TABLE 7                                                         ______________________________________                                                           Crosslinker of                                                                           Crosslinker of                                             CYMEL ®                                                                           EXAMPLE 2  EXAMPLE 2                                                  327     PART A     PART B                                          ______________________________________                                        100  C.                                                                       Mils         1.38      1.33       1.29                                        KHN          7.4       5.1        5.3                                         Pencil       F         HB         F                                           MEK          200       85         140                                         % Removed    2         50         50                                          125° C.                                                                Mils         0.95      0.98       0.97                                        KHN          13.9      14         13.6                                        Pencil       H         H          H                                           MEK          200+      200+       200+                                        % Removed    No Mar    No Mar     No Mar                                      150 ° C.                                                               Mils         0.95      0.95       0.94                                        KHN          15.8      14.2       14.4                                        Pencil       2H        2H         2H                                          MEK          200+      200+       200+                                        % Removed    No Mar    No Mar     No Mar                                      % HCHO Emission                                                                            2.29      1.31       1.49                                        (wt % emitted HCHO                                                            based on cured film)                                                          % HCHO Emission                                                                            7.6       4.4        5.0                                         (wt % emitted HCHO                                                            based on crosslinker)                                                         ______________________________________                                    

EXAMPLE 8

Cure responses and formaldehyde emission studies: Water borneapplication

Formulation:

Joncryl® 540/Crosslinker of Example 1 Weight ratio: 70/30

Solids: 45 wt. %

Surfynol® 104: 1.6 wt. % on TRS

Methoxypropanol: 3.8 wt. % on TRS

Cater: #52 Substrate: B1000 CRS

Flash: 15 min.

Cure: 30 min.

HCHO emission: 150° C./30 min. on glass, humid air, 15 min. flash

                  TABLE 8                                                         ______________________________________                                                           Crosslinker of                                                                           Crosslinker of                                             CYMEL ®                                                                           EXAMPLE 1  EXAMPLE 1                                                  325     PART A     PART B                                          ______________________________________                                        100  C.                                                                       Mils         1.2       1.2        1.3                                         KHN          9.3       8.6        7.4                                         Pencil       F         F          F                                           MEK          200       200        200                                         % Removed    5         2          35                                          125 ° C.                                                               Mils         1.2       1.2        1.2                                         KHN          11.4      11.4       11.9                                        Pencil       H         H          H                                           MEK          200       200        200                                         % Removed    scratch   scratch    slight scratch                              150° C.                                                                Mils         1.2       1.1        1.2                                         KHN          12.2      11.7       11.9                                        Pencil       H         H          H                                           MEK          200+      200+       200+                                        % Removed    No Mar    No Mar     very                                                                          slight scratch                              % HCHO Emission                                                                            3.0       1.8        1.8                                         (wt % emitted HCHO                                                            based on cured film)                                                          % HCHO Emission                                                                            10.0      6.0        6.0                                         (wt % emitted HCHO                                                            based on crosslinker)                                                         ______________________________________                                    

EXAMPLE 9

Cure responses and formaldehyde emission studies: Solvent borneapplication

Formulation:

ACRYLIOD® AT-400/Crosslinker of Example 1 Weight ratio: 70/30

Solids: 66 wt. %

n-BuOH: 20 wt. % on TRS

Solvent: 11.4 wt. % Xylene on TRS

Cater: #34 for 125 and 150° C./30 min. and #52-100° C./30 min.

Cure: 30 min.

                  TABLE 9                                                         ______________________________________                                                           Crosslinker of                                                                           Crosslinker of                                             CYMEL ®                                                                           EXAMPLE 1  EXAMPLE 1                                                  325     PART A     PART B                                          ______________________________________                                        100  C.                                                                       Mils         1.37      1.37       1.38                                        KHN          9.9       8.0        5.3                                         Pencil       F         F          F                                           MEK          200       185        150                                         % Removed    20        50         50                                          125° C.                                                                Mils         0.95      0.98       0.95                                        KHN          15.3      14.9       13                                          Pencil       2H        2H         H                                           MEK          200+      200+       200+                                        % Removed    No Mar    No Mar     No Mar                                      150° C.                                                                Mils         0.91      0.95       0.95                                        KHN          17.8      17.2       15.4                                        Pencil       2H        2H         2H                                          MEK          200+      200+       200+                                        % Removed    No Mar    No Mar     No Mar                                      % HCHO Emission                                                                            2.62%     1.72%      1.00%                                       wt % emitted HCHO                                                             based on cured film)                                                          % HCHO Emission                                                                            8.7       5.7        3.3                                         wt % emitted HCHO                                                             based on crosslinker)                                                         ______________________________________                                    

EXAMPLE 10

Cure responses and formaldehyde emission studies: Waterborne application

Formulation:

Joncryl® 540 acrylic resin/Crosslinker of Example 5 Weight ratio:71.7/28.5

Catalyst: 2.0 wt. % Cycate 4045 cure catalyst based on TRS

Cosolvent: 8.4 wt. % Methoxy Propanol on TRS

Surfynol® 104: 1.5 wt. % on TRS

Solids: 43 wt. % TRS

Hold: 10 min.

Cater: #52

Substrate: B100 CRS

Cure: 20 min.

HCHO Emission: 150° C./30 min. humid air

                  TABLE 10                                                        ______________________________________                                                               CROSSLINKER OF                                                     CYMEL ® 322                                                                          EXAMPLE 5                                              ______________________________________                                        120° C.                                                                Mils          1.2          1.2                                                KHN           8.2          9                                                  MEK           200+         200+                                               % Removed     5            very scratched                                     150 ° C.                                                               Mils          1.2          1.2                                                KHN           9.9          10.8                                               MEK           200+         200+                                               % Removed     very scratched                                                                             No Mar                                             % HCHO Emission                                                                             2.23         1.21                                               (wt % emitted HCHO                                                            based on cured film)                                                          % HCHO Emission                                                                             7.8          4.3                                                (wt % emitted HCHO                                                            based on crosslinker)                                                         ______________________________________                                    

EXAMPLE 11

Cure response and formaldehyde emission: Waterborne application

Formulation: Acryloid® AT 400/Crosslinker of EXAMPLE 4 Weight ratio:70/30

Solids: 66 wt. %

n-BuOH: 15 wt. % on TRS

No Catalyst

Cure: 30 min.

                  TABLE 11                                                        ______________________________________                                                   CYMEL ® 327                                                                          Crosslinker of                                                                EXAMPLE 4 PART A                                        ______________________________________                                        100° C.                                                                Mils         1.05         1.05                                                KHN          12.4         12.7                                                MEK          200+         200+                                                125° C.                                                                Mils         1.00         1.00                                                KHN          14.6         14.3                                                MEK          200+         200+                                                150° C.                                                                Mils         1.00         0.95                                                KHN          15.8         14.3                                                MEK          200+         200+                                                % HCHO Emission                                                                            2.94         1.74                                                (wt % emitted HCHO                                                            based on cured film)                                                          % HCHO Emission                                                                            9.8          5.8                                                 (wt % emitted HCHO                                                            based on crosslinker)                                                         ______________________________________                                    

Although the present invention is described with reference to certainpreferred embodiments, it is apparent that modifications and variationsthereof may be made by those skilled in the art without departing fromthe scope of this invention as defined by the appended claims.

We claim:
 1. A process for preparing a liquid, partiallyalkoxymethylated melamine crosslinker composition containing less than0.5 weight percent free formaldehyde based on 100 percent solids andless than 6.0 weight percent N-substituted methylol groups based on 100percent solids, comprising the steps of:(i) reacting melamine withformaldehyde; (ii) reacting the product from (i) with an etherifyingagent; (iii) reacting the product from (ii) with an etherifying agent;then (iv) reacting the product from (iii) with an etherifying agent,toproduce a liquid partially alkoxymethylated melamine crosslinker producthaving 2.6 to 4.6 moles of combined formaldehyde per mole of melamine,1.4 to 4.4 moles of alkyl per mole of melamine, 1.3 to 3.4 moles of NHper mole of melamine, a free formaldehyde level of less than 0.5 weightpercent based on 100 percent solids, and an N-methylol level of lessthan 6.0 weight percent.
 2. The process of claim 1, wherein theetherifying agent comprises an alcohol and an acid catalyst.
 3. Theprocess of claim 2, wherein the alcohol has 1 to 8 carbon atoms.
 4. Theprocess of claim 3, wherein the alcohol is methanol.
 5. The process ofclaim 1, wherein the product has 2.6 to 3.4 moles of combinedformaldehyde to per mole of melamine, 1.4 to 3.2 moles of alkyl per moleof melamine, and 1.8 to 3.4 moles of NH per mole of melamine.
 6. Theprocess of claim 1, wherein the product has 3.8 to 4.2 moles of combinedformaldehyde to per mole of melamine, 2.6 to about 3.8 moles of alkylper mole of melamine, and 1.4 to 1.8 moles of NH per mole of melamine.7. The process of claim 1, wherein the product has an N-methylol levelof less than 5.0 weight percent based on 100 percent solids.
 8. Theprocess of claim 7, wherein the product has an N-methylol level of lessthan 4.0 weight percent based on 100 percent solids.